This invention relates to data transmission and, more particularly, to high speed data transmission over the Synchronous Optical Network (SONET).
Inverse multiplexers are currently available that enable higher speed signals to be transmitted over available lower speed channels. Specifically, various inverse multiplexers are available in the marketplace that take a contiguous bandwidth signal larger than 56 or 64 Kb/s, and break it down into N parts that can each be transmitted over a 56 or 64 Kb/s DS0 channel. After transmission over T1 transmission facilities, an inverse multiplexer at the receiving end realigns and recombines the N individual signals and outputs the original contiguous bandwidth signal. This process can be called virtual concatenation of N 56 or 64 Kb/s signals and is used to provide a variety of signal bandwidths that are required for data and video transmission but are not available through the public network. Many terminal equipment suppliers offer such equipment in a fast growing market. They all use supplier-proprietary schemes to coordinate the virtual concatenation between the transmitting and receiving ends to manage operation and maintenance. For example, markers are put on the separate signals and are used at the receiving end to properly recombine the signals. Specifically, since the separate signals are likely to be transmitted over different transmission facilities which each impose different transmission delays on the separate signals, the separate received signals must be properly phase aligned at the receiver before they can be recombined. By identifying the marker in each separate received signal, the signals can be phase aligned and recombined in a proper manner. Because of the proprietary schemes used, inverse multiplexers from different suppliers are generally not end-to-end compatible.
In addition to the inverse multiplexers noted above that virtually concatenate N 56 or 64 Kb/s signals, inverse multiplexers are appearing that virtually concatenate N DS1 (1.544 Mb/s) signals that can be used to transmit contiguous bandwidth signals larger than 1.5 Mb/s over multiple T1 transmission lines. Larger bandwidths are again adapted to available channels in the public network. Obviously, the trend is toward higher and higher speed digital transport networks and concomitant with that is the need to transmit at speeds even higher than the networks can accommodate in a single channel. This permits transmission of higher speed contiguous bandwidth signals that do not map into existing channels or that precede the availability of higher speed channels. Thus, even as optical fibers become the transport medium for high-speed digital signals, inverse multiplexing will be required to transmit signals having bandwidths greater than the bandwidth of a single channel.
As optical fibers replace electrical conductors for high-speed digital transport, equipment that complies with adopted SONET standards will be available. SONET standards have been adopted to allow fiber optics transmission systems of one manufacturer to optically interconnect with those of any other manufacturer, and for equipment of different manufacturers to be mixed and matched. SONET standards define standard optical signals, a synchronous frame structure for multiplexed digital traffic, and operations procedures (see, e.g., Rodney J. Boehm, "SONET: A Standard Optical Interface Emerges," Telephony, Apr. 4, 1988, pp. 54-57; Ralph Ballart and Yau-Chau Ching, "SONET: Now It's the Standard Optical Network," IEEE Communications Magazine, March 1989, pp. 8-15; and Yau-Chau Ching and Grant W. Cyboron, "Where is SONET?," IEEE LTS, November 1991, pp. 44-51).
The basic SONET signal, called an STS-1, or Synchronous Transport Signal-Level 1, has a rate of .apprxeq.50 Mb/s. The STS-1 signal defines a multiplexing technique and interface parameters. Each STS-1 signal is a byte-oriented structure that is repeated every 125 msec, with each byte defining a 64 Kb/s channel. The basic signal has a portion of bandwidth set aside for transmission management purposes (transport overhead) that is common among all signals. The other part of the bandwidth (information payload) is designated to be defined in any manner necessary to allow transport of information signals. FIG. 1 shows the format of an STS- 1 frame consisting of 90.times.9=810 bytes. Each frame includes 27 bytes of transport overhead and a Synchronous Payload Envelope (SPE) consisting of 87.times.9=783 bytes of which 774 bytes are payload bytes and 9 bytes are path overhead bytes. Included in the 27 bytes of transport overhead are framing bytes A1 and A2 which define the frame, and pointer bytes H1 and H2 which point to where the SPE begins. The SPE, beginning with the J1 byte in the path overhead, can float anywhere following the H1-H2-H3 overhead bytes. Each frame, defined as beginning with the A1-A2 framing bytes, is therefore not byte contiguous with its SPE, as can be noted in the figure, since the J1 byte can slide from the byte after H3 until the byte before H1 following the next A1-A2 framing bytes.
The ability for the SPE to float is part of the SONET system mechanism incorporated to permit slight variations in phase and frequency to be accommodated as the STS-1 signal is transmitted through the network and passes through many network elements. In particular, as the signal passes through a network element, the SPE and the pointer to the location of that SPE can slide forward or backward by one position to accommodate a phase or frequency variation. After passing through many network elements between a transmitting and a receiving end, therefore, the location of SPE can significantly differ from its original position.
A SONET inverse multiplexer that virtually concatenates SONET signals is envisioned to serve the same purpose as the previously described lower speed inverse multiplexers. The SONET inverse multiplexer will thus break down super-rate signals into multiple STS-1 signals for transport over the current STS-1 based network, or in the future, break down even faster super-rate signals into multiple STS-3c, or faster, signals for transport over a future STS-3c based network. The STS-3c based network has a rate of .apprxeq.155 Mb/s and the STS-3c signal consists of three concatenated STS-1 format signals.
As with currently available lower speed inverse multiplexers, a SONET-speed inverse multiplexer which virtually concatenates plural STS-1 signals (or higher rate signals on an STS-3c or higher speed network) into an original even higher super-rate signal, must include means for realigning the multiple STS-1 signals so they can be properly combined in a byte-by-byte manner. As previously described, as each component STS-1 signal is separately transmitted over different transmission facilities, it will likely encounter different delays that must be compensated for at the receiving end. In a SONET network, transmission of the component STS-1 signals is likely to produce two distinct effects: firstly, the beginning of each frame in each STS- 1 component signal, defined by the A1 and A2 bytes, is likely to be misaligned; and secondly, the SPE in each STS-1 component signal is likely to be offset from its original position, with the offset being different in each separate STS-1 signal.
SONET rate signals could be realigned using marker signals embedded in the payload of each data stream, in the same manner prior art inverse multiplexers realign component signals. Disadvantageously, unless the particular marker signals are standardized, equipment from different suppliers would be incompatible. Furthermore, use of a portion of the payload for transmission of a marker signal decreases the efficiency of information transmission.
An object of the present invention is to realign and combine multiple SONET rate signals at the receiving end of an inverse multiplexer in a manner that does not rely on particular marker signals embedded in the payload of each component data stream, and which is efficient from a transmission standpoint.